Current limiting with a mosfet?

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Is it easy to make a simple circuit to limit a 12V DC current to 50A using a mosfet?

Can I just give it a variable voltage to turn it on a certain amount? Or is it not that easy?

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If it's simpler, a potentiometer to vary the voltage drop across the mosfet would do.

I'm trying to balance a few power supplies all running in parallel, so they do the same work each. I don't mind tweaking a potentiometer on each one to make the current about equal on each.

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You could make a circuit that shuts the FET OFF when you hit 51 amps. If you want to limit current by lowering the output voltage using that FET, then you are asking for blown up FET I think.

You'd need a huge amount of dissipation to make that work and maybe you don't want to reduce the output voltage.

But just shutting off could work fairly easily.

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Yes, series fets could easily fry at 50 amps. Open loop with gate pots would be barbaric.

If the supplies have remote sense, that could be used to balance them.

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Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it's adjusting.

At the moment I'm just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

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Unfortunately they don't (AFAIK). They're old server power supplies. Two of 2600W and two of 1300W HP. There are many pins, and I've heard some can even be used to talk to another supply to balance, but probably not between differing models. If a pin is a remote voltage sense, that would really help, a voltage divider could be used easily to fool it into raising the voltage very slightly. At the moment I've only connected the 0V and 12V power lines, and two of the little pins to power it up (in the same way as you do with an ATX).

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Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it's adjusting.

At the moment I'm just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

• posted

Shutting it off would worsen the problem - the other supplies would then be likely to hit their limits.

Surely the TO-247 MOSFETs can take a high current? Or is that only when turned fully on? I guess it would be complicated to make them pulse, and probably upset the SMPS it's adjusting.

At the moment I'm just going to sort out the biggest imbalance - two of the supplies being 12.85V, two being 12.35V, and one adjustable. The two high voltage ones I can put big TO-220 schottkys on to drop half a volt.

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Sounds iffy; if the potentiometer setting drifts, what happens?

If you really need to gang a 50A+ output, use a design that's meant for paralleling outputs; there's switchmode converters that have provision for multiple paralleled units, by coupling the control sections' duty cycles. Vicor is one source.

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Well, if you have, say, a 1 milliOhm FET which are obtainable, then at

50 amps that is 2.5 watts which can be OK with a good heat sink.

If you were to say, drop 1 volt across the FET at 49 amps, now you are talking a out 1 X 49 = 49 watts which is NOT going to work even with a TO-247 package.

So, you turn off the FET and latch off if you want to limit current and then it only dissipate a few watts at most while fully on.

boB

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Barbaric, but doable for a 1-off. High current fets are free aplenty these days in waste psus, pcs etc OTOH the whole project sounds ill conceived, but what's new.

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A TO-247 on a big heatsink and fan arrangement could easily dissipate 50W.

But I want to LIMIT the current, not fuse it.

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Simplest method: Open up the power supplies and find the resistive divider in the voltage sensing circuit that sets the output voltage. Put a trimpot with series resistor across one of the resistors, to make it slightly adjustable, but with hardly any more range than needed to reach the desired voltage. Then adjust each of the SMPS to the same output voltage. Then, rather than paralleling all of the SMPS with fat wire and running longer wire from there to your load, instead run separate, fairly long (maybe longer than physically necessary) (and equally long for each PSU) wires, no thicker than necessary, from each SMPS separately to the load. This will provide a small ballast resistor between each SMPS and the load. You can still twist the positive and negative output wires of each SMPS, to minimise the loop area between them and thereby minimise inductance. If the inductance of the long wires is still a problem, you could add lots more electrolytic capacitors at the load end.

Proper method: Buy SMPS designed for parallel operation.

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I thought of that, but they're tightly packed complex circuits, and I don't even have a pinout. The power output was obvious, the huge connectors. I had to find the power on pin by trial and error.

They are, but I don't have the specs sheet. They're from an HP blade server, and the end user is supposed to just plug them in and not know what the pins do.

At £14 for 2600W at 12V, I wasn't going to buy anything else!

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OK then.

Use an N-channel FET with a small Ohm resistor in series with its source for the output. 15 milli-Ohms is about right for this one.

Drain goes to your source voltage. 12V here, right ?

Then, tack in a small NPN transistor with its emitter to the output load, its base to the source of the FET and its collector to the gate of the FET. Choose a series R that drops around 0.7 volts at 50 amps and then the NON will turn on, reducing the gate-source voltage and therefore reducing the output current. That's about the simplest that can be done.

You will need to use a resistor driving the gate so that the NPN can easily reduce that Vgs to limit the current.

boB

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That sounds feasible, apart from dropping 0.7 volts. I've got supplies at 12.35V, and supplies at 12.85V. I want to drop the 12.85 to match the 12.35 ones. If they drop below 12.35 I'd have to put something on the 12.35V ones too, and end up with less than 12V on the load, which works best nearer the max of 12.6V rather than the min of 11.4V.

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Interestingly, that config tends to blow out the NPN.

And the fet gate would have to be pullled up to +20ish.

Something with an opamp, and maybe a p-fet, would be more adjustable and run from +12.

0.7 volts and 50 amps across the resistor is 35 watts.
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Yeah, that is a bit much of dissipation I agree. I was trying to show a very cheap way to do this that has worked fine for me.

Doesn't hurt the NPN as long as the drive signal (your +20V) has too low of impedance. Just use a resistor.

But you should NOT need 20V to the gate either. 12V to 16V works fine.

Using an open collector comparator might be an OK way to go EXCEPT that the FET might turn ON slow and burn up if a simple Vgs pull-up resistor is used.

boB

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If the nfet source is at +12, the gate needs to be a bunch more positive.

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OK, right. 12V plus 12V = 24V or so IF it is in the positive leg. Could put it in the negative leg. He didn't say it had to be positie leg but I assumed +12 and a follower.

We use this circuit but that FET is also a voltage regulator using a zener on the gate and a pullup to a plentiful voltage pullup. It was easier in that kind of circuit that has a higher input voltage available.

Not a big deal as it limits current anyway. He wanted to choose a method that would go linear when it started to current limit.

You have to do some homework with this kind of stuff and add whatever is needed to make it work. Or not work in this case.

And that 35 watts will be nothing compared to what that FET would be dissipating when it starts to limit..

boB

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